Union leaders are threatening to step up strike action at Dublin Bus as hundreds of thousands of commuters face disruption on the first day of a number of planned stoppages.

They have also accused Transport Minister Shane Ross of engaging in sound-bites rather than directly intervening to resolve the increasingly bitter row over drivers' pay.

The capital is being hit with two days of traffic gridlock as people turn to private cars and taxis - who are reporting a 300% spike in trade - since buses were called back to their depots on Wednesday evening.

The walkout by members of the National Bus and Rail Union (NBRU) and Siptu is the first of three 48-hour stoppages planned for this month.

It was due to begin at 12.01am on Thursday but the last services ran three hours earlier to ensure engineers and supervisors were in depots as the fleet was locked up.

Dublin Bus said it was a health and safety issue and apologised to customers.

With little sign of any moves towards a resolution, Dermot O'Leary, NBRU general secretary, warned a crunch meeting next Thursday could determine whether the unions will escalate their industrial action.

"People have to get to work and schools and colleges, and certainly the NBRU would be bringing a view to the table that escalation should be the last resort," he said.

"However if Shane Ross, the Government and the company continue to ignore us, the pressure will come on us from our members to escalate."

Willie Quigley, regional officer for the union, blamed "persistent underfunding" of Dublin Bus by successive governments for the fall-out.

"Dublin Bus provides a vital public service, yet we have one of the lowest levels of state support for public transport in the EU," he said.

"In Dublin, the public subvention makes up just 20% of public transport revenue, compared with over 50% in many other European cities."

He added: "Rather than shrugging off responsibility with sound-bites about cheque books, Minister Ross needs to intervene now in this dispute - and he needs to fight the public transport corner at the Cabinet table, making the argument for an increase in subventions to European levels."

Mr Ross had remarked that the Government will not pull out a cheque book to resolve the row.

Buoyed by the successful fight by Luas drivers and inspectors earlier this year for a salary increase, union leaders want a 15% pay increase for Dublin Bus drivers over the coming three years.

Further strikes are planned on September 15 and 16 and again on September 23 and 24, and pickets are being held in Clontarf, Conyngham Road, Harristown, Ringsend, Phibsboro, Donnybrook and Summerhill, as well as at the head office in O'Connell Street.

The shutdown is hitting Airlink services to Dublin Airport, all city services, the Ghostbus Tour on Wednesday, Nitelink buses on Friday and Saturday and all sightseeing services.

Gardai have warned motorists not to use bus lanes even though no buses are running.

Business lobbyists with Retail Ireland said bus users account for about 42% of the money spent in shops in Dublin city.

Director Thomas Burke said: "Cutting off such a vital service will undoubtedly have an impact on city centre footfall and trading levels during the days of industrial action.

"The prospect of ongoing stoppages in services during September is a cause for great concern and must be avoided."

But Owen Reidy, organiser with trade union Siptu, said striking workers are being backed by the public.

"The workers have been humbled by the support shown to them by members of the public on picket lines at bus depots across the city," he said.

"It is clear that a majority of those who use public transport support our members in their demand for fair pay and sufficient State funding for a bus service which is vital to communities across the city."

Mr Reidy said the Department of Transport has yet to give any indication that it will enter talks to resolve the dispute.

"Unfortunately, due to this intransigence our members will be back out on the picket line tomorrow and are prepared for further industrial action until this situation changes," he added.